Thanks for the replies chaps. The car drives fine with no misfires. It doesn't do it all the time, often it will fire up straight away, but you know when its going to play up as it seems to crank over for a few seconds before kicking back against the starter.

I remember the good old days of Mk3 Escorts doing this. I've tried repositioning the crank sensor wire to keep it away from the coil etc, but so far no good.

I will try sticking a strobe on it today though. The biggest problem I have is the girl who owns it doesn't have any money, so the enthusiasm is disappearing fast!I know she hasn't owned the car long, so I don't know its history.